The present invention relates to an image forming system for performing a tandem copying operation in which images are simultaneously formed and output by a plurality of image forming apparatuses on the basis of image data input by one image forming apparatus.
According to a prior-art technique, in an image forming system comprising two image forming apparatuses, image data input from an input section of one of the image forming apparatuses is stored in an image management section of this image forming apparatus and also transferred to and stored in an image management section of the other image forming apparatus via an external interface. On the basis of the image data, images are simultaneously formed and output by the two image forming apparatuses and thus the image formation performance of the whole image forming system is enhanced.
In addition, in the above image forming system, in order to control output numbers of the two image forming apparatuses, there is known a method of assigning initial output numbers to the two image forming apparatuses according to their image formation performances. In this case, if one of the apparatuses does not complete an assigned output number while the other has completed an assigned output number and the output number is not less than a predetermined minimum redistribution number, the output number is redistributed to the two apparatuses, thus forming and outputting images. Thereby, the performance (image formation performance) of the whole system is enhanced.
In this image forming system, however, where image data input from one of the two image forming apparatuses is simultaneously output from the two image forming apparatuses and another image formation output needs to be executed by one of the two image forming apparatuses, a control for said another image forming output is not executed until the output number assigned to said one image forming apparatus is completed. Thus, there arises a problem in that an immediate interrupt cannot be effected.